This invention relates generally to the pressing process for manufacturing composite wood-based panel products. More particularly, it relates to an improved pressing process wherein steam or other suitable condensable hot gas is injected into a wood furnish-adhesive mat during the pressing cycle.
In the manufacture of composite wood-based panel products, wood particles in various forms are combined with thermosetting or sometimes thermoplastic binder systems and formed into loosely compacted mats. The mat is then pressed to final thickness and density under pressure and elevated temperatures while the adhesive is cured. The wood particles can be in fiber form, flake form, particulate form, strand form and other forms that are known in the industry. The generic end products that result are referred to by a variety of names such as fiberboard, hardboard, flakeboard, strandboard, particleboard, and waferboard and indicate the constituent type particulate material within the product. In the case of harboard or medium density fiberboard, there is also an indication of the product density. Each product however, is characterized by being manufactured with wood particulate material and an adhesive system to bind the wood together. These panel products have a variety of well-known end uses.
In a typical manufacturing process, using fiberboard as an example, a refining station reduces the incoming wood raw material to fiber form. The fiber is then dried and directed to a blending station where the thermosetting resin is added in a controlled manner and from there to a forming station where the fiber-resin mixture is formed into loosely compacted mats. The mats can be formed individually atop cauls, although more typically the mat is continuously formed atop a moving supporting structure such as an endless belt. After the mat is formed, it must be compacted and the fiber-resin mixture pressed to thickness and final density at the pressing station. A prepressing station is normally employed to initially reduce the mat thickness and density to manageable levels prior to entry into the final pressing station. Typically, individual mats are then loaded into a platen hot press which is then closed and the resin allowed to cure. More recently single opening, quasi continuous presses have been utilized to press long mats of the wood-resin mixture. The cure time can vary depending upon resin type, final panel thickness, and density, but for a typical medium density fiberboard panel product having a thickness of 19 mm (3/4"), the cure time is approximately 7-8 minutes.
The final board or panel product should have properties falling within the predetermined ranges for all panel characteristics under control. The density should be controlled as should the panel thickness. The surface should be smooth, uniform, and free from blemishes.
In typical prior art pressing systems utilizing hot platens, the resin cure time is determined, in part, by the heat transfer into the mat once the platens compress the mat. Heat must be distributed throughout the fiber-resin mixture in order to bring the entire volume of material up to the desired cure temperature. When only the conductive heat transfer vehicle is utilized, the time required to uniformaly heat the mat and cure the resin is significant.
It has been proposed in the past to use hot gases, such as steam, as a heat transfer medium to bring the unconsolidated or partially consolidated mat temperature up to the desired curing temperature quickly and to reduce consolidation pressures. For example, U.S. Pat. No. 3,280,237 assigned to the assignee of the present invention discloses the use of a superheated steam injection method to improve the pressing process in the manufacture of composite wood panel products. By utilizing superheated steam injected into the porous mat, the cure times were reduced significantly.
The process, as disclosed in U.S. Pat. No. 3,280,237, while having pressing times significantly lower than state-of-the-art press cycles, did not become commercially feasible primarily because of problems with product quality but also because of the requirement for superheated steam which is expensive to generate. It was found that an unacceptable number of panels coming out of the press were affected with blistering, surface pitting, and panel warping. It was determined the blisters were caused by incomplete steam penetration. This effect results in uncured resin and therefore structurally weak or unsound areas in the panel. Such panels are either unacceptable entirely or they must be degraded int a less valuable product going to different end uses.
Surface pitting was found to be caused by the impact of the steam flow as it was injected into the mat through holes in the platen. The mass and velocity of the steam flow was found to disturb the fiber-resin mixture in its uncured form directly under the steam injection holes. Such surface pitting is undesirable and can result in degrading a panel product into a lower grade.
Finally, the panel warping was the result of steam injection from one platen only. This resulted in the panel surfaces not having equal physical properties or uniform moisture levels after pressing. While press cycle time was reduced, the product quality was generally unacceptable and therefore the steam pressing process as disclosed in U.S. Pat. No. 3,280,237 did not become commercially viable. It has a disadvantage the requirement for superheated steam which is expensive as a heat transfer medium in a steam pressing process. Ideally, although not a requirement for practicing the present invention, saturated steam or high quality should be used as its heat of condensation can be used effectively in quickly raising the temperature of the mat and because it is less costly to generate than superheated steam.
While the potential benefits to be derived through the use of hot gases injected into a wood-resin mixture during the pressing cycle were known, a process had not been developed to successfully reduce press cycle times while producing acceptable panels of the desired grade. An improvement in the pressing system was needed to make it commercially feasible for implementation.
Accordingly, from the foregoing, one objective of the present invention is to reduce or eliminate blister, pitting and warping problems when utilizing hot gas injection to reduce press cycle times.
Another objective of the present invention is a methodology to predict the appropriate parameter values for hot gas pressing cycles for panels of various thicknesses and final densities.
These and other objectives of the present invention will become more apparent upon reading the description of the preferred embodiment in conjunction with the attached drawings.